In the production of integrated circuits it is usual to prepare a mask comprising a number of dies, usually several hundred dies arranged in regular rows and columns on a mask. To produce such a mask the design for a single layer of circuit is produced, ordinarily from a drawing, painting, or the like, and ordinarily relatively large in size. At the present time, this circuit usually comprises a number of lines laid down in the proper design and generally may comprise this design in the form of a thin pattern of chromium or other material coated on the surface of a transparent carrier such as a glass slide or the like. The design generally is produced by photo-etch methods.
The first design usually exists in a relatively enlarged size, called a reticle, and is used as the master for projection by means of a step-and-repeat camera onto a transparent surface which will become the mask. Ordinarily, this mask also is formed by photo-etch methods, and the step-and-repeat camera performs the exposure step of such photo-resist procedure to position the dies in precise positions on the mask. Typically, several hundred reduced sized replicas of the original pattern will be formed in precise orientation and location on the mask surface. As a consequence of photo-resist processing there is formed the mask comprising a transparent surface, such as a piece of glass having several hundred dies on its surface, each die corresponding to an exact reduced sized replica of the original reticle.
To produce complete integrated circuits this mask and similar ones are used as masters for laying down individual layers of patterns, one on top of the other in precise position and orientation. Thus, each mask is used in combination with others to form several hundred completed integrated circuits. Such integrated circuits are then employed to form complex electronic equipment. While partial inspection of masks is generally carried out, the complexity is such that many mask defects are not detected during inspection. At the present time, in process inspection is unreliable or overly time-consuming and performance inspection cannot be done until manufacture of the integrated circuit is completed.
Heretofore, the testing of masks has generally been done individually by visual examination. A skilled worked looks at a mask through a microscope, or perhaps at a projection of the microscope image on a CRT display, to detect breaks in the lines of the mask, bulging lines which may touch other lines, or other mask defects. It is estimated that perhaps 25% or more of an entire production force may be engaged in such examination and testing and that such testing may, even so, fail to detect all or even most of the flaws, even though they actually are visible.
As a result of these limitations on inspection, there can be serious inefficiencies or reduced product yield resulting in the waste of many man hours of labor and many hours of wasted time for manufacturing equipment.